BOGOTA (AP) — It's a game played out regularly on the high seas off Colombia's Pacific coast: A U.S. Navy helicopter spots a vessel the size of a humpback whale gliding just beneath the water's surface.

A Coast Guard ship dispatches an armed team to board the small, submarine-like craft in search of cocaine. Crew members wave and jump into the sea to be rescued, but not before they open flood valves and send the fiberglass hulk and its cargo into the deep.

Colombia has yet to make a single arrest in such scuttlings because the evidence sinks with the so-called semi-submersible.

A new U.S. law and proposed legislation in Colombia aim to thwart what has become South American traffickers' newest preferred means of getting multi-ton loads to Mexico and Central America.

Twelve people have been arrested under the Drug Trafficking Vessel Interdiction Act of 2008 since it went into effect in October. It outlaws such unregistered craft plying international waters "with the intent to evade detection." Crew members are subject to up to 15 years in prison.

"It's very likely a game-changer," said Jay Bergman, the U.S. Drug Enforcement Administration's regional director, based in Colombia. "You don't get a get-out-of-jail free card anymore."

The law faces legal challenges, though. The defendants have filed pretrial motions saying it violates due process and is an unconstitutional application of the so-called High Seas clause, which allows U.S. prosecution of felonies at sea.

The vessels, hand-crafted in coastal jungle camps from fiberglass and wood, have become the conveyance of choice for large loads, humping nearly a third of U.S.-bound cocaine northward through the Pacific, said Coast Guard Rear Adm. Joseph Nimmich, commander of the Joint Interagency Task Force-South based in Key West, Fla.

That's up from just 14 percent in 2007, according to the task force, which oversees interdiction south of the United States.

Colombian Navy chief Adm. Guillermo Barrera told a counterterrorism conference in Bogota last week that 23 semi-submersibles capable of carrying between 4 and 10 metric tons each have been seized in the past three years.

Though semi-submersibles aren't new to cocaine transport, a bigger, sleeker, more sophisticated variety that average about 60 feet (18 meters) in length began emerging three years ago. Earlier versions, christened "floating coffins," couldn't compete with fishing trawlers and speed boats known as "go-fasts" for maritime transport of drugs.

But drug agents started policing trawlers better, leading traffickers to new methods.

With just over a foot of above-water clearance and V-shaped prows designed to leave minimal wakes, semi-submersibles are nearly impossible for surface craft to detect visually or by radar outside a range of about 10,000 feet (3,000 meters.)

That accounts for their relatively high success rate.

They are propelled by 250 to 350 horsepower diesel engines and take about a week averaging 7 knots (8 mph) to reach Mexico's shores, Colombian and U.S. investigators said.

Fuel tanks carry about 3,000 gallons of diesel, so no refueling is needed on the 2,000-mile journey from Colombia north.

With cocaine in Mexico fetching $6,500 per kilo — about triple the Colombian price, according to the U.S. Drug Enforcement Administration — an average 7-metric-ton load yields $30 million.

Crews have no problem scuttling the vessels after off-loading their cargo, investigators say. The roughly $1 million spent on each craft is simply written off as the cost of doing business.

Though authorities caught 11 semi-subs last year in international waters off the Pacific — with 7 tons of cocaine seized in one off Mexico in September — they estimate from intelligence and interdiction that another 60 delivered their cargo, Nimmich said.

About the same amount will get through this year, predicts Adm. James Stavridis, the U.S. Southern Command chief. He told a mid-March U.S. Senate hearing they would have a potential cargo capacity of over 330 metric tons.

So far this year, crews sunk five semi-subs off Colombia's coast after being pursued by drug enforcers.

Two of the crews were arrested, plus a third one plucked out of the Pacific on Dec. 31 about 100 miles off Colombia. All are being tried in a Tampa, Fla., federal court, said Joseph Ruddy, the assistant U.S. attorney prosecuting them.

Semi-subs confiscated on land in Colombia since 2007 have given authorities a good glimpse into the state of the art.

In November, authorities arrested a man they consider the most ingenious semi-sub builder. Tammer Portocarrero, a rotund 45-year-old, used a shrimp boat fleet as cover, said Capt. Luis German Borrero, the navy chief in the Pacific port of Buenaventura at the time.

They seized two of his subs at a jungle shipyard in a remote estuary south of Buenaventura, Borrero said.

Portocarrero, whose extradition the United States has requested, allegedly began building vessels as early as mid-2007, as well as recruiting crews.

The made-to-order vessels have become increasingly sophisticated. Engines and exhaust systems are typically shielded to make their heat signatures nearly invisible to infrared sensors used by U.S. and allied aircraft trying to find them.

The cooling system of a semi-sub seized off Costa Rica in September piped engine exhaust through the hull and discharged it at ambient temperature, Nimmich said.

Unfortunately for crews, such design sophistication doesn't extend to their quarters.

"The conditions are terrible," Borrero said. "They don't have bathrooms. The beds are two mattresses draped over the fuel tanks, and the pilot can barely see through very small windows" in mini-cabin.

"The noise and heat must be something infernal," he added.

In a report provided to The Associated Press, Colombia's domestic intelligence agency said a four-person crew was sharing a payoff of about $50,000 per trip before the new U.S. law. Crews now demand about 25 percent more because of the higher risk of getting caught, U.S. law enforcement officials say.

GPS location devices and satellite phones are standard onboard equipment, and the technology is expected to advance.

Law enforcement officials say they already have unconfirmed reports of robotic semi-subs in action.

And with such vessels, Nimmich said, it's not drug smuggling that worries him, but a larger potential for peril:

"I think that what makes semi-submersibles a larger national security threat is: What else can they carry?"